Integrated released beam sensor for sensing acceleration and associated methods

ABSTRACT

An integrated circuit and method are provided for sensing activity such as acceleration in a predetermined direction. The integrated released beam sensor preferably includes a switch detecting circuit region and a sensor switching region connected to and positioned adjacent the switch detecting circuit region. The sensor switching region preferably includes a fixed contact layer, remaining portions of a sacrificial layer on the fixed contact layer, and a floating contact on the remaining portions of the sacrificial layer and having only portions thereof directly overlying the fixed contact layer and in spaced relation therefrom in a normally open position and extending lengthwise generally transverse to the predetermined direction so that the floating contact contacts the fixed contact layer responsive to acceleration in the predetermined direction. The floating contact is preferably a released beam which is released by opening a window or removing unwanted portions of the sacrificial layer. The methods of forming an integrated sensor advantageously are preferably compatible with know integrated circuit manufacturing processes, such as for CMOS circuit manufacturing, with only slight variations therefrom.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuits, and,more particularly, to an integrated circuit and method having thecapability of sensing activity.

BACKGROUND OF THE INVENTION

Over the years, various microelectromechanical systems ("MEMS") havearisen which require the necessity to sense temperature, pressure,strain, acceleration, rotation, infrared radiation, chemical propertiesof liquids and gases, and other physical inputs. Accordingly, varioustypes of microsensors have been developed which receive analog anddigital electrical inputs and also sense or measure these other physicalinputs, e.g., acceleration, pressure, temperature, strain.

Integrated circuits are widely used in many of these MEMS or electronicapplications. Various integrated circuit manufacturing processes, e.g.,very large scale integrated ("VLSI") are also widely known and providevarious advantages. The complimentary metal oxide semiconductor ("CMOS")manufacturing technology, for example, generally provides a low powerdissipation advantage over known metal oxide semiconductor ("MOS")processes. Microsensor manufacturing which is compatible with knownintegrated circuit manufacturing processes, however, can be quitecomplicated, especially because of a need for integrating various typesof structures at relatively low cost.

Examples of applications for microsensors for acceleration oraccelerometers include air bag systems, anti-lock braking systems, andride suspension systems for automobiles and in-flight aircraftmonitoring systems for aircraft. Each of these applications requiressmall, inexpensive, and reliable acceleration devices.

Many of the known accelerometers for these applications, for example,are analog and measure or sense an electrical current that varies withfrequency or amplitude of acceleration. In other words, in essence, manyof these sensors convert mechanical parameters to other energy domainsand then sense or measure directly. For sensors using direct sensing,the parameters are conventionally related to strain, stress, ordisplacement. The principles conventionally used to measure or sensestrain are piezoelectricity, piezoresistivity, and capacitive orinductive impedance.

The measurement of piezoelectric effects, however, often requires a highinput impedance amplifier to measure the surface charges or voltagesgenerated by the stress or the strain. These types of sensors can beexpensive and are often not readily acceptable for high densityintegrated circuit technology and various integrated circuitmanufacturing technology.

The measurement of piezoresistivity in conductors and semiconductorsconventionally involves the strain on the crystal structure deformingthe energy band structure and, thus, changing the mobility and carrierdensity that changes the resistivity or orientation. These type ofsensors, however, are also like piezoelectric sensors in that thesesensors can be expensive to manufacture and often may not be very stablefor acceleration applications.

Capacitive or inductive impedances can also be used to measureacceleration. Examples of such sensors can be seen in U.S. Pat. No.5,417,312 by Tsuchitani et al. titled "Semiconductor Acceleration Sensorand Vehicle Control System Using The Same," U.S. Pat. No. 5,506,454 byHanzawa et al. titled "System And Method For Diagnosing CharacteristicsOf Acceleration Sensor," U.S. Pat. No. 5,610,335 by Shaw et al. titled"Microelectromechanical Lateral Accelerometer," and U.S. Pat. No.5,659,195 by Kaiser et al. titled "CMOS Integrated Microsensor With APrecision Measurement Circuit." Capacitive devices integrate the changeof elementary capacitive areas while piezoresistive devices take thedifference of the resistance changes of bridge arms. Accordingly,capacitive sensors are generally less sensitive to the sideways orindirect forces and are generally more stable. Capacitive sensors,however, conventionally require a capacitance-to-voltage converter on ornear the chip to avoid the effects of stray capacitances which cancomplicate the associated circuitry. The measurement circuitry for thesetypes of sensors is also required to be stable and have low noise.

Additionally, some accelerometers provide a digital output by using a"spring" that either makes or breaks an electrical contact in responseto acceleration. Some of these spring elements, for example, may providea series of sensing elements having incrementally higher responsethresholds which make electrical contact when the threshold is reached.These "spring" accelerometers, however, are relatively large in size ascompared to VLSI circuitry and can be quite difficult to make compatiblewith current integrated circuit manufacturing processes.

Yet further types of microsensors which provide a digital output fordetecting translational or rotational acceleration are also known. Anexample of such a microsersor can be seen in U.S. Pat. No. 5,610,337 byNelson titled "Method of Measuring The Amplitude And Frequency Of AnAcceleration." One type of accelerometer uses a sensing element whichhas some sort of pivotally mounted tilting beam (see FIG. 1B therein).The pivotally mounted tilting beam includes a hinge portion, a rigidconnection member connected to the hinge portion, and a pair ofrespective end portions, e.g., a proof mass, connected to the rigidconnection member. The end portions rotate clockwise and counterclockwise during rotational, e.g., horizontal, movement. Only one of theend portions contacts a contact electrode which responsively stores thecontact signal to indicate that the movement was in the one direction.In essence, this sensing element provides three-states, namely tiltedcontact in one direction, tilted contact in the other direction, oruntilted or neutral. Such a sensing element, however, requires a resetpulse or a reset position which needs to be activated by an externalreset activation source.

Another type of accelerometer which provides a digital output is alsoillustrated in this patent (see FIGS. 2-4). This sensing elementprovides a cantilever beam type arrangement that includes a thick beam,a thinner portion of flexible material connected to and extendingoutwardly from the thick beam and defining a hinge, and a thicker proofmass or end portion connected to and extending outwardly from the hinge.This arrangement of a cantilever beam has problems with "stuck on"conditions which also require complex reset structures and conditions.This arrangement also may include small critical dimension which canmake manufacturing of such a device difficult and expensive with knownintegrated circuit manufacturing processes such as CMOS technology.

SUMMARY OF THE INVENTION

With the foregoing in mind, the present invention advantageouslyprovides an integrated CMOS sensor and associated methods for sensingacceleration. The present invention also advantageously provides anintegrated sensor that is readily compatible with existing integratedcircuit manufacturing technology and manufacturing processes, that hasgreater tolerance for small critical dimensions, and that providesbetter signal indication when interfacing with logic of an integratedcircuit. The present invention additionally provides a cost effectivemethod of forming an integrated sensor for sensing activity desired tobe sensed, such as acceleration in a predetermined direction.

More particularly, the present invention advantageously provides anintegrated sensor for sensing acceleration in a predetermined direction.The integrated sensor preferably includes a switch detecting circuitregion and a sensor switching region connected to and positionedadjacent the switch detecting circuit region. The switch detectingcircuit region is preferably provided by a CMOS switch detecting circuitregion, such as an inverter circuit. The sensor switching regionpreferably includes a fixed contact layer, remaining portions of asacrificial layer on the fixed contact layer, and a floating contact onthe remaining portions of the sacrificial layer and overlying the fixedcontact layer. The floating contact preferably extends lengthwisegenerally transverse to the predetermined direction so that the floatingcontact layer contacts the fixed contact layer responsive toacceleration in the predetermined direction to form a closed switchposition. The switch detecting circuit region preferably generates asignal responsive to contact of the floating contact with the fixedcontact layer.

According to a first embodiment of the present invention, the floatingcontact defines a released cantilever beam. This release cantileverconfiguration, for example, preferably has the remaining portions of thesacrificial layer positioned between a first conducting layer definingthe fixed contact layer and a second conducting layer defining thefloating contact. At least unwanted portions of the sacrificial layerare removed so that the floating contact has only one support at an endthereof defined by the remaining portions of the sacrificial layer andthereby defining a released cantilever beam configuration directlyoverlying the fixed contact layer.

According to a second embodiment of the present invention, the floatingcontact is a released beam overlying the fixed contact layer and havinga configuration which includes a plurality of supports. The plurality ofsupports, for example, can be a double support configuration which alsoforms by having remaining portions of a sacrificial layer positionedbetween a first conducting layer defining the fixed contact layer and asecond conducting layer defining the floating contact. At least unwantedportions of the sacrificial layer are removed, e.g., forming a window,so that the floating contact has at least two supports, e.g., onopposing ends, for the floating contact defined by the remainingportions of sacrificial layer, a window in between the opposingsupported ends directly overlying the fixed contact layer, and therebydefines a double support released beam configuration.

According to other aspects of the present invention, the released beamof the sensor switching region of the integrated sensor preferablyextends outwardly from the switch detecting circuit region a firstpredetermined length. The fixed contact layer extends outwardly from theswitch detecting circuit region a second predetermined length. Thesecond predetermined length is preferably greater than the firstpredetermined length so that the released beam contacts the fixedcontact layer responsive to acceleration in the predetermined directionso as to form the closed switch position.

An integrated sensor according to the present invention preferablyfurther includes acceleration calibrating means associated with thereleased beam for providing a calibrated acceleration sensed by theintegrated sensor. The acceleration calibrating means preferablyincludes a predetermined length of the released beam so as tosubstantially correspond to a selected acceleration calibrationthreshold. The selected acceleration calibration threshold is preferablydefined by a portion of the fixed contact layer substantiallycorresponding to a region of contact of the released beam with the fixedcontact layer.

According to yet other aspects of the present invention, an integratedsensor can also include the sensor switching region having a pluralityof floating layers positioned adjacent and lengthwise extendingoutwardly from the switch detecting circuit region for defining aplurality of released beams so that displacement of each of theplurality of released beams in a predetermined direction corresponds tosensing activity. The plurality of released beams preferably include atleast two released beams lengthwise extending outwardly from the switchdetecting circuit region to different predetermined lengths. Theplurality of beams can also include at least two released beamslengthwise extending outwardly from the switch detecting circuit regionto substantially the same predetermined lengths.

The present invention also advantageously provides methods of forming anintegrated sensor. A method of forming an integrated sensor preferablyincludes providing a switch detecting circuit region and forming asensor switching region connected to and positioned adjacent the switchdetecting circuit region. The sensor switching region is preferablyformed by at least forming a first conducting layer of material on asupport so as to define a fixed contact layer and forming a secondfloating conducting layer overlying the second conducting layer so as todefine a released beam.

According to one aspect of the method, the released beam preferablyforms a released cantilever beam. This method, for example, can includedepositing a sacrificial layer on the first conducting layer, depositinga second conducting layer on the sacrificial layer, and removing atleast unwanted portions of the sacrificial layer, e.g., by etching, torelease the second conducting layer so as to define the releasedcantilever beam.

According to another aspect of the method, the released beam preferablyforms a released beam overlying the fixed contact layer so as to have aconfiguration including at least two supports. This method, for example,can include depositing a sacrificial layer on the first conductinglayer, depositing a second conducting layer on the sacrificial layer,and removing at least unwanted portions of the sacrificial layer, e.g.,by etching, to release the second conducting layer so as to define thereleased beam having the at least two supports.

According to other aspects of the method of forming an integratedsensor, the method can further include forming an insulating layer onthe support prior to the step of forming the first conducting layer. Thefixed contact layer is preferably formed of at least one of polysiliconand a metal, and the second floating conducting layer is also preferablyformed of at least one of polysilicon and a metal.

Therefore, the present invention advantageously provides an integratedsensor and associated methods having a small ship area which allowsarrays of sensors to be fabricated on the same die. The presentinvention also advantageously provides integrated sensors and methodswhich increase the reliability of the sensing of the activity, such asacceleration or deceleration. The fixed contact layer and the floatingcontact of the integrated sensor thereby advantageously provide amicro-mechanical sensing region that can readily be formed with knownintegrated circuit manufacturing processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the features, advantages, and benefits of the present inventionhaving been stated, others will become apparent as the descriptionproceeds when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic block diagram of an integrated sensor having areleased cantilever beam for sensing acceleration and decelerationaccording to a first embodiment of the present invention;

FIG. 2 is an isometric view of a sensor switching region of anintegrated sensor having a released cantilever beam for sensingacceleration and deceleration according to a first embodiment of thepresent invention;

FIG. 3 is a schematic circuit diagram of an integrated sensor having areleased cantilever beam for sensing acceleration and decelerationaccording to a first embodiment of the present invention;

FIG. 4 is a graph of released cantilever beams as a function ofacceleration and beam length for an integrated sensor according to afirst embodiment of the present invention;

FIG. 5 is a schematic block diagram of an integrated sensor having adouble-support released beam for sensing acceleration and decelerationaccording to a second embodiment of the present invention;

FIG. 6 is a schematic block diagram of an integrated sensor having asensor switching region which includes a plurality of released beams forsensing acceleration and deceleration according to a third embodiment ofthe present invention; and

FIG. 7 is a graph of cumulative probability versus number of workingcantilevers of an integrated circuit according to a third embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings which illustrate preferredembodiments of the invention. This invention may, however, be embodiedin many different forms and should not be construed as limited to theillustrated embodiments set forth herein. Rather, these illustratedembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout, andprime or double prime notation, if used, indicates similar elements inalternative embodiments.

FIGS. 1-4 illustrate an integrated sensor 10, a microelectromechanicalintegrated circuit, for sensing acceleration, as well as negativeacceleration or deceleration, in a predetermined direction M, generallyindicated by the arrow along the direction of the force of acceleration,according to a first embodiment of the present invention. The integratedsensor 10 preferably includes a switch detecting circuit region 12 and asensor switching region 20 connected to and positioned adjacent theswitch detecting circuit region 12. The switch detecting circuit region12 is preferably provided by a CMOS switch detecting circuit region suchas an inverter circuit (see, e.g., FIG. 3). The CMOS switch circuitregion may also include a processor 40 connected to the inverter circuitfor processing signals responsive to the inverter. The processor 40, forexample, can be a processing circuit, a logic circuit, or amicroprocessor or microcontroller as understood by those skilled in theart.

The sensor switching region 20 preferably includes a fixed contact layer23, remaining portions of a sacrificial layer 24 on the fixed contactlayer 23, and a floating contact 26 on remaining portions of thesacrificial layer 24 and having portions thereof directly overlying thefixed contact layer 23 in spaced relation therefrom in a normally openedposition. The floating contact 26 preferably overlies the fixed contactlayer 23 and extends lengthwise generally transverse to thepredetermined direction or the predetermined direction of movement M sothat the released beam 26 flexibly contacts the fixed contact layer 23responsive to acceleration in the predetermined direction M to form aclosed switch position. The switch detecting circuit region 12 generatesa signal responsive to contact of the floating contact 26 with the fixedcontact layer 23.

According to the first embodiment of the present invention, the floatingcontact 26 is preferably an integrally monolithic layer of material,e.g., polysilicon or a metal, has a generally uniform thickness theentire length thereof, and defines a released cantilever beam (see FIG.2). This release cantilever configuration, for example, is formed in agenerally vertical direction by having only remaining portions of asacrificial layer 24 positioned between a first conducting layerdefining the fixed contact layer 23 and a second conducting layerdefining the floating contact or floating contact layer 23. The fixedcontact layer 23 is preferably formed on an insulating layer 22 which,in turn, is formed on a substrate 21 or other support. The insulatinglayer 22 is preferably formed of a nitride layer on a field oxide oroxide layer on the substrate 21. At least portions 27 of the sacrificiallayer 24 are removed so that the floating contact 26 advantageously hasonly one support preferably at an end thereof defined by the remainingportions 25 of the sacrificial layer and thereby defining a releasedcantilever beam configuration (see FIG. 1). The released beam 26 flexesor flexibly moves downward due to the force of acceleration on the beamin the predetermined direction.

As perhaps best illustrated in FIG. 5, according to a second embodimentof an integrated sensor 10' of the present invention, the floatingcontact 26' of the sensor switching region 20' is a released beamoverlying the fixed contact layer 23' and has a configuration whichincludes a plurality of supports 25'. The plurality of supports 25', forexample, can be a double or dual support configuration which also formsby having a sacrificial layer 24' positioned between a first conductinglayer defining the fixed contact layer 23' and a second conducting layerdefining the floating contact 26'. At least portions 27' of thesacrificial layer 24' are removed, e.g., forming a window, so that thefloating contact 26' has at least two supports 25', e.g., preferably onopposing ends, for the floating contact defined by the remainingportions of sacrificial layer, a window in between the opposingsupported ends, and thereby defines a double support released beamconfiguration.

This double or dual support released beam configuration will generallyhave different calibration characteristics for length and thickness. Theflexing or moving region of the beam 26', for example, will generallyrequire a lot more, or an order of magnitude greater, the accelerationforce for a corresponding thickness and length of the releasedcantilever beam configuration dependant on certain parameters, such asthe vertical gap or spacing, for example. Nevertheless, the doublesupport configuration, for example, advantageously can reduce potential"stick on" events and reduce overstress or overstrain conditions onportions of the beam 26'. The flexing or moveable portion of the beam 26preferably has a generally uniform thickness the entire length thereofand can also have thicker portions of polysilicon and/or oxide atopposing ends thereof as illustrated.

According to other aspects of the present invention, the released beam26 of the sensor switching region 20 of the integrated sensor 10preferably extends outwardly from the switch detecting circuit region 12a first predetermined length L1 (see FIG. 1). The fixed contact layer 23extends outwardly from the switch detecting circuit region 12 a secondpredetermined length L2. The second predetermined length L2 ispreferably greater than the first predetermined length L1. This greaterlength L2, for example, advantageously can allow a better contact fromthe released beam 26 so that a signal generated by the contact with thefixed contact layer 23 responsive to acceleration in the predetermineddirection more accurately indicates the closed switch position.

FIG. 3 illustrates a schematic circuit diagram for the integrated sensor10 according to the first and second embodiments of the presentinvention. The sensor switching region 20 of the integrated sensor 10preferably operates to provide a digital output such as provided by acantilever switch. The CMOS switch detecting region 12, for example, canbe an inverter which includes a PMOS field effect transistor Q1 and anNMOS field effect transistor Q2. The circuit region 12 can also includea resistor R1, e.g., preferably provided by a poly resistor (e.g., 10¹²Ohms or Teraohms), connected to the gates of the transistors Q1, Q2 ofthe inverter as illustrated.

Operationally, when the input across the resistor R1 is high and theswitch 20 is open, e.g., no contact occurs from the released beam 26 tothe fixed contact layer 23, then the output from the inverter is low.When a force from acceleration in the predetermined direction ofmovement occurs, the switch 20 closes and the low input is received bythe inverter so that the output from the inverter is high.

The integrated sensor 10 also preferably has a relationship betweenreleased cantilever beams as a function of acceleration and beam length.FIG. 4, for example, graphically illustrates the deflection ofpolysilicon cantilever beams as a function of acceleration and beamlengths at 10, 50, and 100 microns. The following Table 1 illustrates anexample of an estimated calculation for this function which was used inthe graph illustrated in FIG. 4:

    ______________________________________                                        Cantilever -- acceleration vs. displacement                                   ______________________________________                                        b = 1 micron        Disp = P*(L.sup.4)/8*E*1                                    H = 0.1 micron P = M*G/b                                                      L = 5, 10, 50, 100 micron I = (b*H.sup.3 /12)                                 G = 1G, 5G, 20G M = (L*b*H) * D                                             ______________________________________                                    

In Table 1, the bulk material properties have been assumed for the YoungModulus. It will also be understood by those skilled in the art that forthe estimated calculations b is the width of the cantilever beam, H isthe height of the cantilever beam, L is the length of the cantileverbeam, G is the acceleration, M is the mass, D is the density, Disp isthe displacement, I is the inertia, and P is the load.

    __________________________________________________________________________    Material                                                                           E    b (width)                                                                          h (height)                                                                         L (length)                                                                         I (Inertia)                                                                        G (acceler)                                                                         D (density)                                                                         M (mass)                                                                           P (load)                                                                           Disp (cm)                                                                          Disp (A)             __________________________________________________________________________    Si   1.50E+12                                                                           1.00E-04                                                                           1.00E-05                                                                           5.00E-04                                                                           8.33E-21                                                                           1000  2.3   1.15E-12                                                                           1.15E-05                                                                           7.19E-12                                                                           0                      Si 1.50E+12 1.00E-04 1.00E-05 5.00E-04 8.33E-21 5000 2.3 1.15E-12                                                                    5.75E-05                                                                      3.59E-11 0                                                                     Si 1.50E+12                                                                  1.00E-04                                                                      1.00E-05                                                                      5.00E-04                                                                      8.33E-21 20000                                                                2.3 1.15E-12                                                                  0.00023 1.44E-10                                                              0                      Si 1.50E+12 1.00E-04 1.00E-05 1.00E-03 8.33E-21 1000 2.3 2.3E-12                                                                     0.000023 2.3E-10                                                              0                      Si 1.50E+12 1.00E-04 1.00E-05 1.00E-03 8.33E-21 5000 2.3 2.3E-12                                                                     0.000115                                                                      1.15E-09 0                                                                     Si 1.50E+12                                                                  1.00E-04                                                                      1.00E-05                                                                      1.00E-03                                                                      8.33E-21 20000                                                                2.3 2.3E-12                                                                   0.00046 4.6E-09                                                               0                      Si 1.50E+12 1.00E-04 1.00E-05 5.00E-03 8.33E-21 1000 2.3 1.15E-11                                                                    0.000115                                                                      7019E-07 72                                                                    Si 1.50E+12                                                                  1.00E-04                                                                      1.00E-05                                                                      5.00E-03                                                                      8.33E-21 5000                                                                 2.3 1.15E-11                                                                  0.000575                                                                      3.59E-06 359                                                                   Si 1.50E+12                                                                  1.00E-04                                                                      1.00E-05                                                                      5.00E-03                                                                      8.33E-21 20000                                                                2.3 1.15E-11                                                                  0.0023 1.44E-05                                                               1438                   Si 1.50E+12 1.00E-04 1.00E-05 1.00E-02 8.33E-21 1000 2.3 2.3E-11                                                                     0.00023 0.000023                                                              2300                   Si 1.50E+12 1.00E-04 1.00E-05 1.00E-02 8.33E-21 5000 2.3 2.3E-11                                                                     0.00115 0.000115                                                              11500                  Si 1.50E+12 1.00E-04 1.00E-05 1.00E-02 8.33E-21 20000 2.3 2.3E-11                                                                    0.0046 0.00046                                                                46000                  Al 7.00E+11 1.00E-04 1.00E-05 5.00E-04 8.33E-21 1000 2.7 1.35E-12                                                                    1.35E-05                                                                      1.81E-11 0                                                                     Al 7.00E+11                                                                  1.00E-04                                                                      1.00E-05                                                                      5.00E-04                                                                      8.33E-21 5000                                                                 2.7 1.35E-12                                                                  6.75E-05                                                                      9.04E-11 0                                                                     Al 7.00E+11                                                                  1.00E-04                                                                      1.00E-05                                                                      5.00E-04                                                                      8.33E-21 20000                                                                2.7 1.35E-12                                                                  0.00027 3.62E-10                                                              0                      Al 7.00E+11 1.00E-04 1.00E-05 1.00E-03 8.33E-21 1000 2.7 2.7E-12                                                                     0.000027                                                                      5.79E-10 0                                                                     Al 7.00E+11                                                                  1.00E-04                                                                      1.00E-05                                                                      1.00E-03                                                                      8.33E-21 5000                                                                 2.7 2.7E-12                                                                   0.000135                                                                      2.89E-09 0                                                                     Al 7.00E+11                                                                  1.00E-04                                                                      1.00E-05                                                                      1.00E-03                                                                      8.33E-21 20000                                                                2.7 2.7E-12                                                                   0.00054 1.16E-08                                                              1                      Al 7.00E+11 1.00E-04 1.00E-05 5.00E-03 8.33E-21 1000 2.7 1.35E-11                                                                    0.000135                                                                      1.81E-06 181                                                                   Al 7.00E+11                                                                  1.00E-04                                                                      1.00E-05                                                                      5.00E-03                                                                      8.33E-21 5000                                                                 2.7 1.35E-11                                                                  0.000675                                                                      9.04E-06 904                                                                   Al 7.00E+11                                                                  1.00E-04                                                                      1.00E-05                                                                      5.00E-03                                                                      8.33E-21 20000                                                                2.7 1.35E-11                                                                  0.0027 3.62E-05                                                               3616                   Al 7.00E+11 1.00E-04 1.00E-05 1.00E-02 8.33E-21 1000 2.7 2.7E-11                                                                     0.00027 5.79E-05                                                              5786                   Al 7.00E+11 1.00E-04 1.00E-05 1.00E-02 8.33E-21 5000 2.7 2.7E-11                                                                     0.00135 0.000289                                                              28929                  Al 7.00E+11 1.00E-04 1.00E-05 1.00E-02 8.33E-21 20000 2.7 2.7E-11                                                                    0.0054 0.001157                                                               115714               __________________________________________________________________________

This graph of FIG. 4 and Table 1 above advantageously illustrate thatvarious lengths of released beams can be used to detect variousthresholds or values of acceleration. For example, when the release beam26 contacts the fixed contact layer 23, such contact will occur only ifa predetermined amount of acceleration force has causes the releasedbeam 26 to deflect to the point of contact with the fixed contact layer23.

An integrated sensor 10 according to the present invention preferablyfurther includes acceleration calibrating means associated with thereleased beam 26 for calibrating acceleration sensed by the integratedsensor 10. The acceleration calibrating means preferably includesforming a predetermined length L1 of the released beam 26 so as tosubstantially correspond to a selected acceleration calibrationthreshold (see also FIG. 4). The selected acceleration calibrationthreshold is preferably defined by a portion of the fixed contact layer23 substantially corresponding to a region of contact of the releasedbeam 26 with the fixed contact layer 23. In other words, the length fora preselected thickness of the released beam 26 is preferablypre-calculated so that contact with the fixed contact layer 23 by thereleased beam 26 only occurs when the force due to acceleration reachesa predetermined threshold or a predetermined value. This contact, e.g.,a closed switch position, is then detected by the switch detectingcircuit region 12, e.g., by the inverter circuit so that the output ishigh (see FIG. 3). The length of the released beam 26, for example,advantageously can vary so that when a plurality of these switchingsensors are used, various acceleration thresholds are detected.

As perhaps best illustrated in FIGS. 6-7, according to yet anotherembodiment of the present invention, an integrated sensor 10" preferablyincludes a switch detecting circuit region 12" and a sensor switchingregion 20" connected to and positioned adjacent the switch detectingcircuit region 12". The sensor switching region 20" includes a pluralityof floating contacts 26" positioned adjacent and lengthwise extendingoutwardly from the switch detecting circuit region 12" for defining aplurality of released beams so that displacement of each of theplurality of released beams 26" in a predetermined direction correspondsto sensing activity.

The integrated sensor 10", also advantageously has a relationshipbetween cumulative probability and the number of working releasedcantilever beams of an integrated sensor 10". FIG. 7, for example,graphically illustrates this relationship. The following Table 2, forexample, provides an illustration of estimated calculations for thisrelationship as graphically illustrated in FIG. 7:

    __________________________________________________________________________    Number                                                                            nCr                                                                              Cum (nCr)                                                                           C10  Number                                                                            nCr  Cum (nCr)                                                                           C7                                           __________________________________________________________________________      1 10 10 0.977517 1 7 7 5.511811                                               2 45 55 5.376344 2 21 28 22.04724                                             3 120 175 17.10655 3 35 63 49.6063                                            4 210 385 37.63441 4 35 98 77.16535                                           5 252 637 62.26784 5 21 119 93.70079                                          6 210 847 82.7957 6 7 126 99.2126                                             7 120 967 94.5259 7 1 127 100                                                 8 45 1012 98.92473                                                            9 10 1022 99.90225                                                            10 1 1023 100                                                               __________________________________________________________________________      Number nCr Cum (nCr) C15 Number C15 C10 C7                                  __________________________________________________________________________      1 15  15 0.045778 1 0.045778 0.977517 5.511811                                2 105 120 0.366222 2 0.366222 5.376344 22.04724                               3 455 575 1.754814 3 1.754814 17.10655 49.6063                                4 1365 1940 5.920591 4 5.920591 37.63441 77.16535                             5 3003 4943 15.0853 5 15.0853 62.26784 93.70079                               6 5005 9948 30.35981 6 30.35981 82.7957 99.2126                               7 6435 16383 49.99847 7 49.99847 94.5259 100                                  8 6435 22818 69.63713 8 69.63713 98.92473                                     9 5005 27823 84.91165 9 84.91165 99.90225                                     10 3003 30826 94.07636 10 94.07636 100                                        11 1365 32191 98.24213 11 98.24213                                            12 455 32646 99.63073 12 99.63073                                             13 105 32751 99.95117 13 99.95117                                             14 15 32766 99.99695 14 99.99695                                              15 1 32767 100 15 100                                                       __________________________________________________________________________          % confidence                                                                              % confidence                                                                              % confidence                                    __________________________________________________________________________    5/7   >93.70 7/10 >94.53 10/15                                                                              >94.076                                           6/7 >99.21 8/10 >98.92 11/15 >98.242                                          7/7 >100.00 9/10 >99.90 12/15 >99.631                                           10/10 100.00 13/15 >99.951                                                      14/15 >99.997                                                                 15/15 100.000                                                           __________________________________________________________________________

The graphical illustration of FIG. 7 and Table 2 advantageously show thecumulative probability of an accurate detection, e.g., "% confidence",or read for the contact of the released beam 26" with the fixed contactlayer 23". For example, if 5 out of 7, 7 out of 10 or 10 out of 15 ofthe released cantilever beams 26" "work" or switch closed, then at leasta 90% confidence level exists that the acceleration threshold has beendetected or the event has occurred. In other words, statisticallyprobability of an event occurring can advantageously be used for formingan integrated sensor 10" having a sensor switching region 20" whichincludes a plurality of these released beams 26" formed as describedherein.

According to aspects of this embodiment of the present invention, theplurality of released beams 26" preferably include at least two releasedbeams 26" lengthwise extending outwardly from the switch detectingcircuit region 12" to different predetermined lengths and at least tworeleased beams 26" lengthwise extending outwardly from the switchdetecting circuit region 12" to substantially the same predeterminedlengths (see FIG. 6). The sensor switching region 20" can furtherinclude at least one fixed contact layer 23' underlying the plurality ofreleased beams 26" so that at least one of the plurality of releasedbeams 26" contacts the at least one fixed contact layer 23" duringsensing of movement in a predetermined direction M so as to form aclosed switch position. The sensor 20" preferably has a plurality offixed contact layers 23" spaced apart from each other generally in thesame plane and has insulating layers or insulating material positionedbetween each of the spaced apart fixed contact layers 23" (See FIG. 6).Like the integrated sensor 10, 10' of the first and second embodiments,the integrated sensor 10" of this embodiment also preferably has theswitch detecting circuit region 12" being responsive to the contact ofthe at least one of the plurality of released beams 26" with the atleast one fixed contact layer 23" and preferably includes a processor40" as described above herein.

As illustrated in FIGS. 1-7, the present invention also advantageouslyprovides methods of forming an integrated sensor 10. A method of formingan integrated sensor 10 preferably includes providing a switch detectingcircuit region 12 and forming a sensor switching region 20 connected toand positioned adjacent the switch detecting circuit region 12. Thesensor switching region 20 is preferably formed by at least forming afirst conducting layer of material on a support so as to define a fixedcontact layer 23 and forming a second floating conducting layeroverlying the second conducting layer so as to define a released beam26.

According to one aspect of the method, the released beam 26 preferablyforms a released cantilever beam. This method, for example, can includedepositing a sacrificial layer 24 on the first conducting layer 23,depositing a second conducting layer 26 on the sacrificial layer 24, andremoving at least portions 27 of the sacrificial layer 24, e.g.,preferably by dry isotropic etching or an oxide release etch asunderstood by those skilled in the art, to release the second conductinglayer so as to define the released cantilever beam 26. This isotropicetch forms concave surfaces in the remaining portions of the sacrificiallayer, including surface or surfaces underlying the released beam 26.This isotropic etch capability, for example, advantageously providesreleasing of the beam 26 so that the overall integrated sensor 10 canreadily be formed with known CMOS processes. Oxide (e.g., S_(i) O₂) orother passivation material is also deposited on the second conductinglayer, and either an isotropic or anisotropic etch used for the oxideoverlying the second conducting layer, e.g., polysilicon.

According to another aspect of the method, the released beam 26'preferably forms a released beam overlying the fixed contact layer 23'so as to have a configuration including at least two supports 25'. Thismethod, for example, can include depositing a sacrificial layer 24' onthe first conducting layer 23', depositing a second conducting layer 26'on the sacrificial layer 24', and removing at least portions 27' of thesacrificial layer 24', e.g., by etching, to thereby defined a window andto thereby release the second conducting layer so as to define thereleased beam having the at least two supports 25'. A nitride mask, forexample, can be used in the process.

According to other aspects of the method of forming an integrated sensor10, the method can further include forming an insulating layer or plate22, e.g., preferably provided by a nitride layer or a nitride layer on afield oxide, on the support plate 21 prior to the step of forming thefirst conducting layer 23. The nitride layer, for example,advantageously protects the underlying field oxide during the releaseetch. If the polysilicon pads are sufficiently layer, however, then thenitride layer may not be needed over the field. The fixed contact layer23 is preferably formed of at least one of polysilicon and a metal, andthe second floating conducting layer 26 is also preferably formed of atleast one of polysilicon and a metal. Both the fixed contact layer andthe released beam are preferably formed of polysilicon material so thatthey can advantageously be readily formed when other CMOS switchdetecting circuitry is being formed, e.g., a continuous process, whereinother polysilicon material is conventionally used. Both the fixedcontact layer and the released beam are also connected to the switchdetecting circuit region 12 as illustrated in FIGS. 1-3 and 5-6.

For example, in forming the integrated sensor 20 according to a methodof the invention, a wafer or substrate can be prepared and an initialfield oxide layer for a target thickness. A nitride layer can then bedeposited on the oxide layer. Most of the CMOS switch detecting circuitregion 12 is preferably formed prior to the sensor switching region 20because more is involved. These processes can include etching andimplant using masks as understood by those skilled in the art. Thesensor switching region can then be prepared and formed in process withthe switch detecting circuit region 12. The first polysilicon layer canbe deposited, and a layer of oxide, e.g., the sacrificial layer 24,deposited on the first polysilicon layer. The second polysilicon layercan then be deposited and, for example, also used for a localinterconnect, a resistor, and the beam 26. Additional, preparation ofthe switch detecting region can be performed, e.g., implanting,depositing, and etching, and then the window in the sacrificial layercan be etched, e.g., by using a high frequency vapor oxide release etchor an isotropic dry vapor oxide release etch, so that the cantileverbeam 26 or the double support beam 26' remain floating.

A method of sensing an activity is also provided according to thepresent invention. The method preferably includes providing a switchdetecting circuit region 12" and providing a plurality of floatingcontacts 26" positioned in spaced relation in a normally open positionwith at least one fixed contact layer 23". Each of the plurality offloating contacts 26" preferably has substantially the same length. Themethod also includes contacting less than all of the plurality offloating contacts 26" with the at least one fixed contact layer 23" soas to form a closed switch position and generating an activityconfirmation signal responsive to a majority of the plurality offloating contacts 26" contacting the at least one fixed contact layer23".

As illustrated and described herein, the integrated sensor 10, 10', 10"of the present invention advantageously provides an integrated sensorand associated methods having a small chip area which allows arrays ofsensors to be fabricated on the same die. The present invention alsoadvantageously provides integrated sensors and methods which increasethe reliability of the sensing of the activity such as the accelerationand deceleration. The fixed contact layer 23, 23', 23" and the floatingcontact 26, 26', 26" of the integrated sensor 10, 10', 10" therebyadvantageously provide a micro-mechanical sensing region 20, 20', 20"that can readily be formed with known integrated circuit manufacturingprocesses as understood by those skilled in the art.

The various embodiments of the integrated sensor 10, 10', 10" and itsassociated methods, including methods of forming the same, may also beadvantageously used for other applications as well. For example, otherintegrated circuitry having related structures are illustrated in thefollowing copending patent applications: (1) "Integrated Sensor HavingPlurality Of Released Beams For Sensing Acceleration And AssociatedMethods," having attorney work docket number 19844, having U.S. Ser. No.08/957,809, assigned to the assignee of the present invention, and theentire disclosure of which is incorporated herein by reference in itsentirety; (2) "Integrated Released Beam Oscillator And AssociatedMethods," having attorney work docket number 18981, having U.S. Ser. No.08/957,804, assigned to the assignee of the present invention, and theentire disclosure of which is incorporated herein by reference in itsentirety; and (3) "Integrated Released Beam, Thermo-Mechanical Sensorfor Sensing Temperature Variations And Associated Methods," havingattorney work docket number 18979, having U.S. Ser. No. 08/957,802,assigned to the assignee of the present invention, and the entiredisclosure of which is incorporated herein by reference in its entirety.

In the drawings and specification, there have been disclosed a typicalpreferred embodiment of the invention, and although specific terms areemployed, the terms are used in a descriptive sense only and not forpurposes of limitation. The invention has been described in considerabledetail with specific reference to these illustrated embodiments. It willbe apparent, however, that various modifications and changes can be madewithin the spirit and scope of the invention as described in theforegoing specification and as defined in the appended claims.

That which is claimed:
 1. An integrated sensor comprising:a switchdetecting circuit region; and a sensor switching region connected to andpositioned adjacent said switch detecting circuit region, said sensorswitching region including;a plurality of floating contacts positionedadjacent and lengthwise extending outwardly from said switch detectingcircuit region for defining a plurality of released beams so that eachof said plurality of released beams displaces in a predetermineddirection responsive to acceleration, said plurality of released beamsincluding at least two released beams lengthwise extending outwardlyfrom said switch detecting circuit region to different predeterminedlengths and at least two released beams lengthwise extending outwardlyfrom said switch detecting circuit region to substantially the samepredetermined lengths, at least one fixed contact layer underlying saidplurality of released beams, remaining portions of at least onesacrificial layer positioned between said at least one fixed contactlayer and each of said plurality of released beams, each of saidplurality of released beams being positioned in spaced relation fromsaid at least one fixed contact layer in a normally open position, thespaced relation forming by removal of unwanted portions of thesacrificial layer, said remaining portions of the at least onesacrificial layer including at least one concave surface underlying eachof said plurality of released beams, and each of said plurality ofreleased cantilever beams contacting said at least one fixed contactlayer responsive to acceleration in a predetermined direction to form aclosed switch position, and at least one insulating support plateunderlying said at least one fixed contact layer, and wherein saidswitch detecting circuit region generates a signal responsive to thecontact of said at least one of said plurality of released beams withsaid at least one fixed contact layer.
 2. An integrated sensor asdefined in claim 1, further comprising acceleration calibrating meansassociated with said released cantilever beam for providing a calibratedacceleration sensed by the integrated sensor.
 3. An integrated sensor asdefined in claim 2, wherein said acceleration calibrating means includesa predetermined length of said released cantilever so as tosubstantially correspond to a selected acceleration calibrationthreshold.
 4. An integrated sensor as defined in claim 3, wherein theselected acceleration calibration threshold comprises a portion of saidfixed contact layer substantially corresponding to a region of contactof said released cantilever beam with said fixed contact layer.
 5. Anintegrated sensor as defined in claim 1, wherein the predeterminedlength of the outward extent of said released cantilever beam comprisesa first predetermined length, wherein said fixed contact layer extendsoutwardly from said CMOS switch detecting circuit region a secondpredetermined length, and wherein the second predetermined length isgreater than the first predetermined length.
 6. An integrated sensor asdefined in claim 1, wherein said insulating support layer comprises anitride layer formed on a field oxide.
 7. An integrated sensor asdefined in claim 1, wherein the sacrificial layer comprises an oxidelayer, and wherein the released cantilever beam comprises an integrallymonolithic material having a generally uniform thickness extendingsubstantially the entire length thereof.
 8. An integrated cantileversensor for sensing acceleration in a predetermined direction, theintegrated sensor comprising:a CMOS switch detecting circuit region; anda sensor switching region connected to and positioned adjacent andextending outwardly from said switch detecting circuit region, saidsensor switching region including;an insulating support layer, a fixedcontact layer positioned on the insulating support layer, remainingportions of a sacrificial layer positioned on portions of said fixedcontact layer, the sacrificial layer comprising an oxide layer, and afloating contact layer on said remaining portions of the sacrificiallayer and having portions thereof directly overlying said fixed contactlayer and in spaced relation therefrom in a normally open position andextending lengthwise generally transverse to the predetermined directionfor defining a released cantilever beam, the released cantilever beamcomprising an integrally monolithic material having a generally uniformthickness extending substantially the entire length thereof, the spacedrelation forming from removal of unwanted portions of the sacrificiallayer, said released cantilever beam contacting said fixed contact layerresponsive to acceleration in the predetermined direction to form aclosed switch position, and wherein said CMOS switch detecting circuitregion generates a signal responsive to contact of said releasedcantilever beam with said fixed contact layer.
 9. An integrated sensoras defined in claim 8, wherein said fixed contact layer comprises atleast one of polysilicon and a metal.
 10. An integrated sensor asdefined in claim 9, wherein said released cantilever beam comprises atleast one of polysilicon and metal.
 11. An integrated sensor as definedin claim 10, wherein said released cantilever beam extends outwardlyfrom the switch detecting circuit region a predetermined length, andwherein said remaining portions of the sacrificial layer includes atleast one concave surface underlying said floating contact layer.
 12. Anintegrated sensor as defined in claim 11, further comprisingacceleration calibrating means associated with said released cantileverbeam for providing a calibrated acceleration sensed by the integratedsensor.
 13. An integrated sensor as defined in claim 12, wherein saidacceleration calibrating means includes a predetermined length of saidreleased cantilever so as to substantially correspond to a selectedacceleration calibration threshold.
 14. An integrated sensor as definedin claim 13, wherein the selected acceleration calibration thresholdcomprises a portion of said fixed contact layer substantiallycorresponding to a region of the contact of said released cantileverbeam with said fixed contact layer.
 15. An integrated sensor as definedin claim 14, wherein the predetermined length of the outward extent ofsaid released cantilever beam comprises a first predetermined length,wherein said fixed contact layer extends outwardly from said CMOS switchdetecting circuit region a second predetermined length, and wherein thesecond predetermined length is greater than the first predeterminedlength.
 16. An integrated sensor as defined in claim 15, wherein saidinsulating support layer comprises a nitride layer formed on a fieldoxide.